Influence of ball-milled amorphous B powders on the sintering process and superconductive properties of MgB2
The sintering process of MgB2 superconductors with milled amorphous B as precursor powder was investigated using thermal analysis, and the corresponding superconductive properties were measured. It was found that the presence of a B2O3 layer on the surface of milled B particles could effectively hinder the solid-solid reaction between Mg and B during the sintering process. Since the formation of MgB2 phase mainly completes after Mg melting, the size and homogeneity of MgB2 grains mainly depends on that of the B particles. As a result, the milling treatment of B powder could result in the formation of refined and homogeneous MgB2 grains, which are responsible for the excellent Jc performance in the prepared sample with briefly milled B powder. However, as the milling time was increased, the B2O3 impurity in the B original powder increased and resulted in an obvious increase of MgO impurity in the prepared sample. These excess MgO impurities led to a significant decrease in Jc at low fields in the sample prepared with the heavily milled B powder. On the other hand, they could also serve as effective flux pinning centers due to their nanometer size and lead to an improvement in the J c of the samples prepared with the heavily milled B powder. 2009 IOP Publishing Ltd.